Coherent detection of a digital message burst in TDMA communications systems requires an estimation of carrier phase and symbol timing of the received transmission. Carrier phase synchronization for TDMA transmissions may typically be provided at the receiver either by gated-tracking over successive bursts of the desired signal or by independent acquisition of carrier phase and symbol timing for each of the separate bursts.
Frequency instability in either the transmitter or receiver portions of the system may not allow the frame-to-frame coherence of carrier phase that is necessary for the gated tracking technique, in which case the carrier phase and symbol timing estimate is updated for each TDMA frame when the signal burst is received. Systems which independently acquire the carrier phase for each received transmission burst require a carrier synchronization preamble at the beginning of each burst. The use of such preambles results in a loss of frame efficiency, inasmuch as time and satellite power is utilized for purposes other than message transmission.
The TDMA/QPSK (Quaternary Phase Shift Keyed) system developed by Kokusai Denshin Denwa Company acquires carrier synchronization independently from burst to burst without the requirement of a carrier preamble. See K. Nosaka, A. Ogawa, and T. Muratani, "PSK-Demodulator with delay line for PCM-TDMA system", Proceedings of 1969 International Communications Conference, Boulder, Colo., pages 37:19-254. In that design, a temperature controlled co-axial cable of 100 meters in length was utilized for a highly precise delay of 25 symbol intervals. The 50Mbs modem obtained carrier synchronization from the undelayed IF signal in order to coherently demodulate the delayed replica of the suppressed carrier QPSK signal. Because of the cost and complexity of the IF delay implementation, this method may be impractical for most TDMA systems.